Ambartsumian’s Knot A dwarf elliptical galaxy in the constellation Ursa Major located at the end of a what appears to be a bridge of matter extending from the elliptical galaxy NGC 3561B (R.A. 11h 11m, Dec. +28° 42'). The latter is interacting with the nearby spiral galaxy NGC 3651A giving rise to a number of tidal tails and tidal dwarf galaxies.

*The entire system is called Arp 105 or, colloquially, The Guitar, because of its overall shape. The enormously long tidal tail visible to the left (north) of NGC 3651A in the accompanying photo, Arp 105N, stretches out for 100,000 parsecs (325,000 light-years) from the parent galaxy. Ambartsumian's Knot lies at the end of a shorter tail called Arp 105S.

Arp 150 lies close to the center of the rich cluster Abell 1185, which lies about 400 million light-years away.

*Arp 105 - NGCs 3561 & 3561A - 3561 m14.7 sb 14 .7'x.7' SBO - 3561A m14.3 .9'x.9' Sa 11h11.3m +28°41'. Arp Classification - E and E-like galaxies connected to spirals. This is a very nice area to get lost in for a few hours. These two galaxies are on the eastern side of Abell 1185, a cluster of 52 galaxies. In the 16" at 300x I had 24 galaxies in the field with others on the bare edge of visibility. 3561 is a small round object about .6' in diameter with a brighter core. 3561A is 1' to the N of 3561, small and oval, aligned NS about .6'x.3' in size. It has a bright almost stellar nucleus. There are very faint wings of nebulosity extending out about .3' on either side of the oval region. On the Arp photograph there is a thin stream of material extending N for about 3' ending in a small condensed object called Ambartsumian's Knot. This was not visible in either the 16" or 20" scopes with which I observed this object. Extending to the W from these two for about 12' is a dense stream of small dim galaxies with a couple of brighter NGC galaxies as outliers on the N and S of this stream. You can actually follow a pretty dense trail of galaxies for about 1.2° to the W of 3561 & 3561A. Immediately to the E of the Arp pair is another stream of faint galaxies, this time extending NS for about 20'. A very interesting field. Dr. Arp thought enough of this object and its immediate environs that he used it for the back cover of his most recent book, "Seeing Red" (Aperion, 1998.)